California regulators are threatening to revoke permits for on-demand ride companies UberX, Lyft, Sidecar, Summon and Wingz unless they stop giving rides to and from airports within two weeks.

The move could lead to the state shutting down the companies’ operations.

“We have heard numerous complaints that (our) safety rules are being ignored,” Michael Peevey, president of the California Public Utilities Commission, wrote on Wednesday in an unusually stern letter to the transportation network companies, or TNCs.

The companies’ citizen drivers give paid rides to and from airports in San Francisco, Oakland, San Jose, Los Angeles and San Diego without permission from the airports, he wrote. Taxi and limo services are required to pay for permits to operate at airports. Peevey cited an SFO report about TNC drivers, mainly affiliated with UberX, lacking airport permits, as well as lacking proper “trade dress” on their vehicles and lacking proof of insurance.

Flouting the airport rules also flouts regulations that the CPUC set up for the new generation of ride companies to operate in California. In a clear rebuttal to an argument often made by the ride companies, Peevey wrote: “These safety requirements should not hinder your creativity nor should they impede your innovation.”

Uber said in a statement that it has been working for months to obtain airport permits.

“We have spent countless hours with airport managers and their staff to help develop requirements that are aligned with CPUC regulations and embrace the spirit of ride-sharing,” said spokeswoman Eva Behrend. “The tone of the majority of these conversations has been mutually respectful and productive. It’s unfortunate the CPUC is not allowing the airport process they designed to proceed by allowing ride-sharing companies to continue working with California airport authorities on their permitting process.”

Peevey said his staff will start random audits of the ride companies’ operations. Enforcement actions, including revoking their permits, will occur “if immediate action is not taken to bring your operation (and the actions of your contractors) into compliance,” he wrote.

If the companies continued to operate after their permits were revoked, the CPUC could not only fine them “but also work with law enforcement to ensure a company ceases operations,” said CPUC spokeswoman Terrie Prosper in an e-mail.

California was the first state to create an operating framework for the smartphone-summoned ride companies, issuing rules in September about driver background checks, driver training, car inspections and insurance.

Uber and Lyft are both rapidly expanding their operations, fueled by huge amounts of venture capital. They’ve faced strong opposition from both local regulators and taxi drivers who see their business disrupted.

On Tuesday, the PUC issued a proposed rule to expand insurance requirements for the companies, including covering the time while drivers wait to be matched with a ride.